The subject disclosure relates to superconducting devices, and more specifically, to fabricating a vertical transmon qubit device using a silicon-on-metal (SOM) substrate.
Quantum computing is generally the use of quantum-mechanical phenomena for the purpose of performing computing and information processing functions. Quantum computing can be viewed in contrast to classical computing, which generally operates on binary values with transistors. That is, while classical computers can operate on bit values that are either 0 or 1, quantum computers operate on quantum bits that comprise superpositions of both 0 and 1, can entangle multiple quantum bits, and use interference.
Quantum computing hardware can be different from classical computing hardware. In particular, superconducting quantum circuits generally rely on Josephson junctions, which can be fabricated in a semiconductor device. A Josephson junction generally manifests the Josephson effect of a supercurrent, where current can flow indefinitely across a Josephson junction without an applied voltage. A Josephson junction can be created by weakly coupling two superconductors (a material that conducts electricity without resistance), for example, by a tunnel barrier.
One way in which a Josephson junction can be used in quantum computing is by embedding the Josephson junction in a superconducting circuit to form a quantum bit (qubit). A Josephson junction can be used to form a qubit by arranging the Josephson junction in parallel with a shunting capacitor. This arrangement of a Josephson junction in parallel with a shunting capacitor is sometimes referred to as a transmon (which is a shortened version of the phrase transmission line shunted plasma oscillation qubit) in the case where the shunting capacitor has a large capacitance such that the typical ratio of the Josephson energy to the charging energy in the qubit is larger than 10. While in some scenarios, where the ratio of Josephson energy to charging energy in the qubit has a smaller ratio may not be referred to as a transmon, herein, a transmon can designate any arrangement of a Josephson junction in parallel with a shunting capacitor. Other superconducting qubits exist that are not transmon qubits.
A transmon generally has a reduced sensitivity to charge noise compared to some other types of qubits. A mechanism by which a transmon can reduce sensitivity to charge noise is by increasing a ratio of Josephson energy to charging energy.
A problem with some prior art transmon qubits is that they occupy a relatively large amount of space. Specifically, a planar capacitor used in some transmon qubits occupies a large area. The compactness of such transmon qubits is limited by both surface and dielectric loss.
Then, there are also problems with some types of prior art Josephson junctions, as applied to producing transmons from these Josephson junctions. A type of Josephson junction with both low loss and low critical current can be made from shadow evaporated aluminum, aluminum oxide, and aluminum (Al—AlOx—Al). However, a problem with such a Josephson junction is that, once the Josephson junction is formed, the resulting device is subject to a low-temperature and process constraint.